Impact of the Cut-Off Voltage on Cyclability and Passive Interphase of Sn-Polyacrylate Composite Electrodes for Sodium-Ion Batteries

Reversibility of electrochemical sodiation for Sn-based electrodes consisting of Sn powder, graphite, and sodium polyacrylate was examined at different upper cutoff voltages of 0.65 and 0.70 V in nonaqueous Na cells. The upper cutoff voltage is one of the key factors to improve the electrochemical reversibility. In case of a cutoff voltage of 0.70 V, the sodiation/desodiation cycle performance was not stable and accompanied by capacity decay, indicating that the anodic decomposition of passivation layer is led to the dissolution and reformation at 0.68 and 0.40 V, respectively, on Sn particles that were catalyzed by pure Sn metal. The repeated dissolution and reformation brought a thicker and resistive surface layer, resulting from the accumulation of electrolyte decomposition products, which was clarified by X-ray photoelectron spectroscopy. In contrast, the capacity retention and stability were improved by simply changing the upper cutoff voltage to 0.65 V due to exclusion of the SEI decomposition at 0.68 V. The results of time of -flight secondary ion mass spectroscopy measurements suggests that the surface passivation layer containing polymer/oligomer on the Sn electrode was successfully formed and enhanced the SEI functionality for 0.65 V cutoff. The Sn-based electrode delivered similar to 700 mAh g(-1) reversible capacity over 100 cycles.